1. Field of the Invention
The present invention relates to a magnetic element and dc-to-dc converter for use in a power supply in an electronic device such as a personal computer, a portable information device, and an electronic exchanger.
2. Description of the Related Art
In recent years, a more compact, low-profile electronic device represented by a personal computer, a portable information device, or an electronic exchanger has been developed. This can be mainly achieved by one-chip electronic circuits obtained by the high-density integration of semiconductor integrated circuits. However, in such a device, a power supply unit for supplying a power has not been downsized. Therefore, the power supply unit begins to be a large factor which retards downsizing of the device and a decrease in device cost.
At present, a power supply using a switching process is used as a power supply for electronic communication equipment. When different voltages are used in the circuits of an electronic device, a dc-to-dc converter is used. In order to decrease the size and weight of the electronic device, the switching frequency of a switching power supply including the dc-to-dc converter has been gradually increased. That is, if the switching frequency is increased, the entire size of the device can be decreased because passive elements such as magnetic elements (inductor or transformer) and capacitors can be decreased in size. As a result, at present, a power supply which can be used in a several 100 kHz band has been realized. In addition, a power supply having a power density (power per unit volume of power supply) of higher than 1 W/cm.sup.3 has been developed.
As a smoothing capacitor of the constituent elements of the dc-to-dc converter, a large-capacitance multilayered ceramic capacitor is developed in place of a conventionally used electrolytic capacitor. Therefore, a compact, low-profile dc-to-dc converter having higher reliability is expected.
On the other hand, in a magnetic element, a planar inductor or transformer is developed to decrease the thickness of the magnetic element. A case wherein the magnetic element is applied to a power supply is reported (e.g., the November 1992 IEICE transactions, T. Sato et al., Vol. E75-B, No. 11, pp. 1186-1191, November 1992). A planar inductor 4, as shown in FIG. 1, has a structure in which a planar coil 1 is sandwiched by amorphous magnetic thin sheets 3 through insulating layers 2, and the planar inductor 4 has a size of 11.times.11.times.0.8 mm.sup.3. The planar inductor has characteristics, i.e., an inductance of 30 .mu.H and a coil resistance of 0.65.OMEGA., and is used at a frequency of several 100 kHz. When the planar inductor is used, a low-profile power supply can be expected.
FIG. 2 shows a buck (step-down) chopper type dc-to-dc converter. A dc-to-dc converter having an output power of 10 W or less is frequently used. In such a dc-to-dc converter, since a ratio of a non-load loss which is not related to the output power to all losses is high, the efficiency of the dc-to-dc converter is generally about 70%. The non-load loss is generated by a PWM control IC 32 for performing constant-voltage control and its external parts (mainly, resistors and capacitors), a driver circuit (a gate driver circuit in a power MOSFET 35, or a base driver circuit in a bipolar power transistor) for a main switching element, and an overcurrent detector. Therefore, when a small-capacity power supply is used, the efficiency of a power semiconductor element, a choke coil, and transformer directly related to power conversion must be improved, and the losses of the peripheral circuits must be reduced.
Of these peripheral circuits, overcurrent detecting means are shown in FIGS. 3 and 4. The overcurrent detecting means shown in FIG. 3 is obtained by inserting a current detection resistor (R) in series with a load. This means advantageously has a simple structure. However, since the means detects a voltage drop which is in proportion to the resistance, the loss of the means increases in proportion to its square of output current. The overcurrent detector shown in FIG. 4 uses a current transformer and FETs. In this structure, an impedance inserted in the circuit can be advantageously decreased. However, in this circuit, a power loss is generated during a steady state operation of a power supply, and the efficiency of the power supply may be decreased. In addition, the current transformer, the FETs, and the like must be added, thereby increasing the number of parts.
At present, the operation voltage of an LSI tends to decrease, a power supply having a low-voltage and large-current output is required. For this reason, the loss of an overcurrent detector tends to increase. In this manner, in a compact, low-profile power supply, an overcurrent detecting means having a small loss is required. However, an effective means for solving the above problem has not yet been known.